F. Tessele*, L.O. Monteggia** and J. Rubio*** *Instituto de Pesquisas Hidra´ulicas. Universidade Federal do Rio Grande do Sul. Av. Bento Gonc¸alves 9500, Agronomia, CP 15029, CEP 91501970, Porto Alegre, RS, Brazil (E-mail:
[email protected]) **Instituto de Pesquisas Hidra´ulicas. Universidade Federal do Rio Grande do Sul. Av. Bento Gonc¸alves 9500, Agronomia, CP 15029, CEP 91501970, Porto Alegre, RS, Brazil (E-mail:
[email protected]) ***Departamento de Engenharia de Minas. Universidade Federal do Rio Grande do Sul. Av. Osvaldo Aranha 99/512 CEP 90035190, Porto Alegre, RS, Brazil (E-mail:
[email protected]) Abstract Post-treatment of an UASB reactor effluent, fed with domestic sewage, was conducted using twostage flotation and UV disinfection. Results were compared to those obtained in a parallel stabilisation pond. The first flotation stage employed 5–7.5 mg L21 cationic flocculant to separate off more than 99% of the suspended solids. Then, phosphate ions were completely recovered using carrier flotation with 5–25 mg L21 of Fe (FeCl3) at pH 6.3–7.0. This staged flotation led to high recoveries of water and allowed us to separate organic matter and phosphate bearing sludge. The water still contained about 1 £ 102 NMP/100 mL total coliforms, which were removed using UV radiation to below detection levels. Final water turbidity was , 1.0 NTU, COD , 20 mg L21 O2 and 71 mNm21, the liquid/air interfacial tension. This flotation-UV flowsheet was found to be more efficient than the treatment in the stabilisation pond and appears to have some potential for water reuse. Results were discussed in terms of the biological, chemical and physicochemical mechanisms involved. Keywords Advanced flotation; municipal wastewater; UASB reactor; wastewater treatment
Water Science & Technology Vol 52 No 1-2 pp 315–322 Q IWA Publishing 2005
Treatment of municipal wastewater UASB reactor effluent by unconventional flotation and UV disinfection
Introduction
In warm climate countries, the high rate anaerobic process presents satisfactory treatment performance, even for diluted domestic wastewater, with many advantages, including reduction of green house gas emissions, energy gains, reduced excess sludge productions, stabilised sludge, and low space requirements (van Lier and Huibers, 2004). In particular, the upflow anaerobic sludge blanket (UASB) reactor is a reliable and simple technology for domestic sewage treatment (van Haandel and Lettinga, 1994). Despite all those advantages, anaerobic processing cannot be considered as an unitary (one-step) treatment system, since its effluent requires further stages to improve water quality enough to reach discharge or reuse standards required by the Brazilian environmental standards (Chenicharo and Machado, 1998). Anaerobic systems effluents may still contain residual organic matter, nutrients and pathogens, which must be removed in a post-treatment stage (van der Steen et al., 1999). In the last decade, the use of high rate anaerobic reactors integrated to pond systems has achieved increasing relevance for domestic wastewater treatment in Brazil (Luduvice et al., 2000; Cavalcanti, 2003). When efficient pre-treatment is used, the concentrations of organic matter and suspended solids are reduced to levels whereby the pond works more like a maturation pond, leading to bacterial decay (van Haandel and Lettinga, 1994).
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In spite of this satisfactory pathogens decay, in ponds, the ever increasing presence of algae and residual phosphate in the final effluent might reduce the efficiency due to eutrophication effects (Mara et al., 2001). Hence, the more accepted flowsheet for simultaneous algae and phosphate removal appears to be based on physicochemical methods, with emphasis on separation by dissolved air flotation using a phosphate carrier (Simmonds, 1973; Bare et al., 1975; Edzwald, 1993; Monteggia and Tessele, 2001; Tessele et al., 2004a). Post-treatment of UASB reactors effluent using flotation
The conventional coagulation, flocculation and dissolved air flotation (DAF), using FeCl3 and cationic polymer, presents fairly high efficiency in improving water quality from anaerobic reactor effluents (Penetra et al., 1999; Reali et al. 2001). Yet, this procedure results in significant volumes of mixed organic and inorganic sludge which may lead to complex post-sludge treatment either to reuse or dispose it (Tessele et al., 2004b). This work presents an alternative for the sustainable domestic wastewater treatment, combining optimised biological and physicochemical processes. When nutrient recovery is considered, the proposed two-stage flotation process brings the advantage of the production of two separated sludges, one containing biomass and the second containing a mixture of ferric phosphate and hydroxide. Results proved that, following this alternative flowsheet (selective process), it was possible to obtain fairly good quality water with low organic matter and phosphate in separated sludge. Experimental set up
The study was conducted, during 20 months, in a pilot plant (50 m3 d21), treating domestic wastewater provided by the municipal sanitation company (DMAE) at Porto Alegre (South Brazil). The studied flowsheet (Figure 1) comprises an up flow anaerobic sludge blanket reactor, a stabilisation pond, two-stage flotation (including the novel FF process and a high rate DAF-dissolved air flotation) and a final UV disinfection stage. The pilot plant integrating all processes started up in September 2002. Operational parameters, weekly monitored, for the biological steps (UASB and stabilisation pond), were: pH, ORP, alkalinity, temperature, COD, NH4-N, PO4, TSS (Standard Methods, 1995). Faecal and total coliforms were measured twice a month. The flotation tests were performed in a semi-continuous regime, with running times of approximately 6 h. Table 1 summarises general data of the staged process and their functions. The two-stage flotation system
The two-stage flotation system was designed to treat the UASB effluent up to 50 m3 d21. Both columns were constructed in 0.5 mm acrylic, with 0.25 m diameter and 4 m high.
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Figure 1 Schematic flowsheet of the pilot-scale wastewater treatment plant. Stabilisation pond runs as a parallel and alternative process to the two-stage flotation (TSF) system
Table 1 Pilot-scale wastewater treatment system General data
Function
Applied loadings
By-product
Anaerobic reactor
UASB, 15 m3
COD removal, rough disinfection
0.8 kg COD m23 d21
Stabilised anaerobic sludge and biogas
Stabilisation pond (SP)
300 m2, 0.8 m depth
COD, NHþ 4 -N and pathogens removal
21 21 36 kg NHþ d 4 -N ha
Atmospheric emissions and bottom sludge
Flotation 1 (F1)
4 m height, 0.25 m diameter, 0.03 m2 section
Algae and TSS removal
120 kg SST m2 d21 HLR ¼ 65 m h21
Organic sludge (anaerobic biomass)
Flotation 2 (F2)
4 m height DAF, 0.25 m diameter, 0.04 m2 section
Phosphate removal
9.7 kg PO4 m2 d21 HLR ¼ 51 m h21
Inorganic sludge FePO4 þ Fe(OH)3
Ultraviolet system
Medium pressure, high intensity
Oxidation and disinfection
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Step
No external by-products
The first stage was to remove suspended solids by the FF (flocculation –flotation) process. This is a novel flotation technique, which was originally developed for oil removal (Rubio, 2003, Rosa and Rubio, 2005). The basis is the formation of aerated flocs, in the presence of high molecular weight polymer under high shear. The second-stage flotation was used to remove phosphate ions by precipitation and coagulation with Feþ3 (FeCl3) and also as a polishing step, separating the residual fine solids. The removal of phosphate ions proceeds through adsorbing colloidal (Fe(OH)3) flotation (ACF) adjusting medium pH (after FeCl3 addition) between 5.5 and 6.5. Results and discussion
The raw wastewater feeding the pilot plant was quite diluted, due to infiltrations of urban drainages on the collection network. Average COD was around 90 mg O2 L21, with peaks in order of 400 mg O2 L21 during dry periods. The UASB reactor was inoculated with 1% p/p sludge from a gelatine manufacturer wastewater treatment. UASB reactor operation
Because the pilot plant started up during spring (average water temperature of 20 8C), COD removal by the UASB reactor was noticeable in the very first days of operation (Figure 2). Yet, gas production was observed after approximately 60 days of operation.
Figure 2 Chemical oxygen demand removal efficiency in the anaerobic reactor (UASB)
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Stabilisation pond performance
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During summer time (water temperature ranging between 28 and 33 8C) the stabilisation pond presented significant removal of ammoniacal nitrogen (Figure 3), giving values below 3 mg L21 for NH4-N. In those hot days, the pH increased to the range 10–11, resulting in elevated reductions of NH3 concentration via gas stripping (Powers, 1987). Figure 3 allows to conclude that in sub-tropical regions, where temperature changes are very common, stabilisation ponds may not be as reliable as expected for NH4-N removal. In winter time, when average water temperatures may reach 17 8C, alkalinity production decays due to the decrease in algae activity (less CO2 consumption), leading to pH values in the range 6–7.
Two-stage flotation (TSF) system operation
The TSF system was operated in semi-continuous regime, during 6–8 hours each day. Process efficiency was monitored in situ by turbidity. During the experiments, chemicals concentrations varied from 0 to 25 mg L21 and 0 to 15 mg L21, for Feþ3 and flocculant, respectively. In the FF process (F1), the air flow rate was about 3 NL min21. In DAF (F2), recycle ratio was kept at 15% (volume ratio), at a saturation pressure 4.5 kgf cm22 and air flow rate of 0.9 –1.2 NL min21. Flotation 1 produced a high solid concentration (up to 11%) sludge which is mainly composed of anaerobic biomass and flocculant. Rising velocities were found to be very rapid with the formed aggregates very resistant to shear, allowing the system to attain 65 m h21 hydraulic loading rate, with residence time of 3.8 min, including the coagulation zone. The flocculation-flotation system (FF) is composed of a turbulent “flocculator” to generate aerated polymeric flocs which feeds, in the present case, a column solid/liquid separation device. The basic concept is that of a contact reactor (zigzag flocculator) and a separator. The resulting flocs are rapidly formed inside the flocculator, are very light because of the trapped air and are generated only in the presence of high molecular weight polymers, bubbles (from the injected air), high shearing forces and a high head loss (Rosa, 2002, Rubio, 2003, Rosa and Rubio, 2005). Main mechanisms and phenomena involved in the FF technique include: small bubble formation and their rapid occlusion (entrapment) within flocs; nucleation of bubbles at floc/water interfaces; and bubbles entrainment. The aerated flocs looked like filamentous, elongated and “sticky” (plastic-like) units, and in the flocculator, plug flow type of mixing (flocculation) instead of perfect has been
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Figure 3 Ammoniacal-nitrogen concentration and pH changes in the stabilisation pond
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Figure 4 Comparative COD values in the staged treatment. RW ¼ raw wastewater; UASB ¼ after UASB reactor; SP ¼ after stabilisation pond; F1 ¼ after flotation 1; F2 ¼ after flotation 2 and UV ¼ after UV disinfection
observed. In the present flotation column separator the flocs float within seconds, as large units (some millimetres in diameter) having very low densities. The second flotation stage (F2) aimed at removing both phosphate and the residual fine solids. The phosphate ions were co-precipitated and adsorbed onto the carrier colloidal ferric hydroxide precipitates which are formed from the hydrolysis of Feþ3 from the FeCl3. Optimal coagulant dosage was dependent on the initial phosphate concentration and amounted to 15 mg L21 Feþ3 and medium pH 6.3–7 (no pH adjustment was required). The phosphate ions might either precipitate as ferric phosphate with the soluble ferric ions or adsorb by chemical interaction with the ferric surface sites (adsorbing colloidal flotation). Coagula and precipitates are fragile aggregates, requiring mild, non-turbulent hydrodynamic conditions at the solid –liquid separation. This is accomplished using dissolved air flotation, with microbubbles (30 –80 microns diameters) (Rubio et al., 2002, Rodrigues and Rubio, 2003). DAF was performed in a column and the process hydraulic loading rate reached about 49 m h21, or 2 minutes residence time. This loading capacity is much higher than conventional, (rectangular) DAF circuits (6 –10 m h21). After flotation 2 (F2), the effluent was disinfected with a low pressure UV lamp, operated at constant conditions, with a theoretical UV dosage of 25 mJ cm22 (according to the manufacturers). The results obtained (Figures 4– 9) shows that the UASB-TSF-UV combination is more efficient than the UASB-SP option. Thus, the produced water presents low values in COD, phosphate ion concentration, turbidity and the air/water surface tension is as high as that of tap water. The removal of ammoniacal nitrogen was not satisfactory accordingly to the Brazilian emission standards (Brasil, 1986). However, when water reuse is taken into account,
Figure 5 Comparative NH4-N values in the staged treatment. RW ¼ raw wastewater; UASB ¼ after UASB reactor; SP ¼ after stabilisation pond; F1 ¼ after flotation 1; F2 ¼ after flotation 2 and UV ¼ after UV disinfection
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Figure 6 Comparative soluble PO4 values in the staged treatment. RW ¼ raw wastewater; UASB ¼ after UASB reactor; SP ¼ after stabilisation pond; F1 ¼ after flotation 1; F2 ¼ after flotation 2 and UV ¼ after UV disinfection
Figure 7 Comparative turbidity values in the staged treatment. RW ¼ raw wastewater; UASB ¼ after UASB reactor; SP ¼ after stabilisation pond; F1 ¼ after flotation 1; F2 ¼ after flotation 2 and UV ¼ after UV disinfection
Figure 8 Comparative interfacial tension values in the staged treatment. RW ¼ raw wastewater; UASB ¼ after UASB reactor; SP ¼ after stabilisation pond; F1 ¼ after flotation 1; F2 ¼ after flotation 2 and UV ¼ after UV disinfection
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Figure 9 Comparative UV254 values in the staged treatment. RW ¼ raw wastewater; UASB ¼ after UASB reactor; SP ¼ after stabilisation pond; F1 ¼ after flotation 1; F2 ¼ after flotation 2 and UV ¼ after UV disinfection
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Figure 10 Comparative total and faecal coliforms in the staged treatment. RW ¼ raw wastewater; UASB ¼ UASB reactor; SP ¼ after stabilisation pond; F1 ¼ after flotation 1; F2 ¼ after flotation 2 and UV ¼ after UV disinfection
many uses are allowed with the average concentration reached (,20 NH4-N mg L21). The possible uses of the treated wastewater would be in agricultural and landscape irrigation, groundwater recharge, environmental and recreational uses, industrial water, washing of, among others, vehicles, tanks, hangars (US EPA, 2004). Coliforms concentrations (Figure 10) were below the emission standards and its removal may be optimised via the UV dosage adjustment, if needed for more strict uses. Conclusions
The 20 months operation cycle showed that the proposed staged process, treating an UASB effluent, produced, at high hydraulic rates, good quality water and two different sludges: the first containing organic matter and the second containing mainly phosphates. The flotation techniques were very efficient (high loading rates) for the solid/liquid separation (high split) and nutrient removal. In addition, the scheme proposed here yielded better water quality, when compared to the stabilisation pond effluent and appears to have a good potential in domestic water treatment and reuse. The possible uses of the treated wastewater would be in agricultural and landscape irrigation, groundwater recharge, environmental and recreational uses, industrial water, among others. Acknowledgements
The authors would like to thanks to all those institutions supporting this research in Brazil. Also, thanks to the students Aline Fronza, Ana P. Heglert, Diego de Oliveira and Vinı´cius Ely for their contribution to the experimental work and discussion of the results.
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Luduvice, M., Neder, K. and Pinto, M.T. (2000). Utilizac¸a˜o de lagoas rasas no po´s-tratamento de efluentes de reatores anaero´bios de fluxo ascendente (UASB). Po´s-Tratamento de efluentes de reatores anaero´bios – Coletaˆnea de artigos te´cnicos vol 1, FINEP/PROSAB pp. 43 –56. Mara, D., Pearson, H., Oragui, J., Arridge, H. and Silva, S.A. (2001). Research Monograph No. 5: Development of a new approach to waste stabilization pond design. University of Leeds, Leeds, UK. Monteggia, L. and Tessele, F. (2001). Remoc¸a˜o fı´sico-quı´mica de algas e fo´sforo de efluentes de lagoas de alta taxa. In: Chenicharo, C. Po´s-Tratamento de efluentes de reatores anaero´bios – Coletaˆnea de artigos te´cnicos – vol. 2, 97 – 102, FINEP/PROSAB, in Portuguese. Penetra, R.G., Reali, M.A.P., Foresti, E. and Campos, J.R. (1999). Post-treatment of effluents from anaerobic reactor treating domestic sewage by dissolved-air flotation. Wat. Sci. Tech., 40(8), 137 –144. Powers, S.E. (1987). Modeling and aerated bubble ammonia stripping process. JWPCF, 94, 92 – 100. Reali, M.A., Penetra, R.G. and Carvalho, M.E. (2001). Flotation technique with coagulant and polymer application applied to the post treatment of effluents from anaerobic reactor treating sewage. Wat. Sci. Tech., 43(8), 205 – 212. Rodrigues, R.T. and Rubio, J. (2003). New basis for measuring the size distribution of bubbles. Miner. Eng., 16(8), 757 –765. Rosa, J.J. (2002). Tratamento de efluentes oleosos por floculac¸a˜o pneuma´tica em linha e separac¸a˜o por flotac¸a˜o – Processo FF. PhD Thesis, PPGEM-Universidade Federal do Rio Grande do Sul, Porto Alegre – Brazil, 145p. (in Portuguese). Rosa, J.J. and Rubio, J. (2005). The FF (flocculation – flotation) process. Miner. Eng., in press. Rubio, J., Souza, M.L. and Smith, R.W. (2002). Overview of flotation as a wastewater treatment technique. Miner. Eng., 15(3), 139 – 155. Rubio, J. (2003). Unconventional flocculation and flotation. In: Flotation and Flocculation: From Fundamentals to Applications, Proceedings from Strategic Conference and Workshop, Hawaii, 2002, Edited by J. Ralston, J. Miller and J. Rubio, pp. 17 – 32. Simmonds, M.A. (1973). Experience with algal blooms and the removal of phosphorus from sewage. Wat. Res., 7, 255 – 264. Standard Methods for the Examination of Water and Wastewater (1995). 19th edn, American Public Health Association/American Water Works Association/Water Environment Federation, Washington DC, USA. Tessele, F., Monteggia, L.O. and Rubio, J. (2004a). Selective nutrient and water recovery from municipal wastewater combining advanced treatment techniques. International Conference on Wastewater Treatment for Nutrient Removal and Reuse. Pathumthani, Thailand, January 26 – 29, vol. 1, pp. 180 – 187. Tessele, F., Da Rosa, J. and Rubio, J. (2004b). Avanc¸os da flotac¸a˜o no tratamento de a´guas, esgotos e efluentes industriais – Parte I: Fundamentos e mecanismos. Saneamento Ambiental, n. 102 Jan/Fev, 30 – 36. US EPA (2004). U.S. Environmental Protection Agency – EPA/625/R-04/108 September 2004 Guidelines for Water Reuse. Van der Steen, P., Brenner, A., Joost, M., van Buuren, M. and Oron, G. (1999). Post-treatment of UASB reactor effluent in an integrated duckweed and stabilization pond system. Wat. Res., 33(3), 615 – 620. Van Haandel, A.C. and Lettinga, G. (1994). Anaerobic Sewage Treatment; A Practical Guide for Regions with a Hot Climate, John Wiley and Sons, Chichester, England. Van Lier, J.B. and Huibers, F.P. (2004). Agricultural use of treated wastewater: the need for a paradigm shift in sanitation and treatment. Risk Assetment of Re-use on Groundwater Quality. Proceedings of symposium HS04 held during IUGG2003 at Sapporo, Japan, July. 2003. IAHS Publ. 285.
